Abstract

Purpose

A review of the role of the carotenoids, lutein and zeaxanthin, and their function
in altering the pathogenesis of age-related macular degeneration (AMD).

Methods

Medline and Embase search.

Results

Recent evidence introduces the possibility that lutein and zeaxanthin, carotenoids
found in a variety of fruits and vegetables may protect against the common eye disease
of macular degeneration. This potential and the lack to slow the progression of macular
degeneration, has fueled high public interest in the health benefits of these carotenoids
and prompted their inclusion in various supplements. The body of evidence supporting
a role in this disease ranges from basic studies in experimental animals to various
other clinical and epidemiological studies. Whilst some epidemiological studies suggest
a beneficial role for carotenoids in the prevention of AMD, others are found to be
unrelated to it. Results of some clinical studies indicate that the risk for AMD is
reduced when levels of the carotenoids are elevated in the serum or diet, but this
correlation is not observed in other studies. Published data concerning the toxicity
of the carotenoids or the optimum dosage of these supplements is lacking.

Conclusion

An intake of dietary supplied nutrients rich in the carotenoids, lutein and zeaxanthin,
appears to be beneficial in protecting retinal tissues, but this is not proven. Until
scientifically sound knowledge is available we recommend for patients judged to be
at risk for AMD to: alter their diet to more dark green leafy vegetables, wear UV
protective lenses and a hat when outdoors. Future investigations on the role of nutrition,
light exposure, genetics, and combinations of photodynamic therapy with intravitreal
steroid (triamcinolone-acetonide) injections hold potential for future treatment possibilities.

Keywords:

Introduction

The two major carotenoids in the human macula and retina are lutein and zeaxanthin
[1,2]. Similar to β-carotene, these pigments are found in various coloured fruits and green
leafy vegetables. Of the 40 to 50 carotenoids typically consumed in the human diet
[3,4], lutein and zeaxanthin, are deposited at an up to 5 fold higher content in the macular
region of the retina [1] as compared to the peripheral retina. Zeaxanthin is preferentially accumulated in
the foveal region [2,5], whereas lutein is abundant in the perifoveal region. These pigments are collectively
referred to as the macular pigment (MP). Although the role of the macular pigment
remains uncertain, several functions have been hypothesised and these include limitation
of the damaging photo-oxidative effects of blue light through its absorption [6-8], reduction of the effects of light scatter and chromatic aberration on visual performance,
[9,10], and protection against the adverse effects of photochemical reactions because of
the antioxidant properties of the carotenoids [5,11,12].

Age related macular degeneration (AMD) is the leading cause of irreversible vision
loss in the elderly population in the USA and the Western world. It is estimated that
1.6% of the population in the 50-to 65-year-old age group is affected, rising to 30%
in the over-75-year-old age group. As the proportion of the elderly in our population
increases, the public health impact of AMD will become even more severe Although the
aetiopathogenesis of AMD remains a matter of debate, there is a growing body of evidence
to indicate that oxidative damage plays a role [13,14]. Consequently, the possibility that the absorption characteristics and antioxidant
properties of macular pigment confer protection against age-related macular degeneration
has been postulated [12,15] and it has further been hypothesised that dietary supplementation with lutein and
/ or zeaxanthin might protect the retina and / or delay the progression of age-related
macular degeneration [12,16-18]. Supplementation with foods [19-21] or supplements rich in lutein or zeaxanthin has been reported to increase macular
pigment density in most, but not all, human subjects [19,21,22]. Yet despite the possible importance of these carotenoids in modulating the course
of age-related macular degeneration, critical evidence of beneficial effect has not
been found, and the role that these carotenoid supplements play in patients with age-related
macular degeneration, or those at risk of developing the disease, remains unproved.
In this article we review the current literature present on carotenoids and focus
particularly on the controversial evidence that retinal carotenoids are protective
against age-related macular degeneration.

Methods

Medline and Embase search.

Results

Age related macular degeneration

Definition and grading

In 1995 the International Age Related Maculopathy (ARM) Study Group published the
international classification and grading system for age related maculopathy and age
related macular degeneration [23]. In this article all age related macular changes are referred to as age related maculopathy
(ARM). ARM is a degenerative disorder involving the retinal pigment epithelium, choriocapillaries
and retina which primarily, but not exclusively, affects the macular region. Symptoms
of ARM include metamorphosia, impaired light adaptation and decreased central vision.
Age related macular degeneration (AMD) is a term reserved for the late stages of ARM
[23]. AMD has been categorised in two forms, an exudative form characterised by subretinal
haemorrhage, detachment of retinal pigment epithelium (RPE), choroidal neovascularization
(CNV), or retinal scarring and a "dry" form which includes geographic atrophy. Once
patients have reached the late stage of this disease, vision loss cannot be restored
[24] and low vision aids are the only known help.

AMD remains the leading cause of legal blindness in the elderly population in the
Western world [25-27]. The risk for AMD has been strongly linked to greater age [28], positive family history of AMD [29], smoking [30,31], female gender [32], and finally the high intake of saturated fat has been demonstrated to be related
to early ARM as shown in the Beaver Dam Eye Study [33]. Reports vary on the relationship between light iris colour and the severity of AMD
or increase risk of for ARM [34-37]. It is estimated that 1.6% of the population in the 50-to 65-year-old age group is
affected, rising to 30% in the over-75 year-old age group [38].

Dietary sources of lutein and zeaxanthin

Lutein is a common carotenoid found in most fruits and vegetables, while zeaxanthin
is present only in minute quantities in most fruits and vegetables [39,40]. Dietary sources of zeaxanthin are limited to greens, certain yellow/ orange fruits
and vegetables such as corn, nectarines, oranges, papaya and squash. Orange pepper
is recently found to have a high amount of zeaxanthin [40] and the dried fruit of Lycium barbarum (fructus lycii) prescribed by the Chinese
herbalist as a therapeutic agent for a number of eye diseases, has been shown to have
a high content of zeaxanthin but negligible amount of lutein [41].

The highest mole percentage of both lutein and zeaxanthin can be found in egg yolk
and maize [40]. Substantial amounts of lutein are also known to be found in melon, spinach [41], collards, kale [42], and guava. In 1998, the United States department of Agriculture (USDA) updated their
Carotenoid database for foods to include the concentrations of lutein and zeaxanthin
in the most common fruits and vegetable consumed. http://www.nal.usda.gov/fnic/foodcomp/Data/car98/car98.htmlwebcite.

The pharmaceutical industry also provides patients with various brands of ocular vitamin
supplement products all claiming superiority over their competitors. Some of the brands
of these ocular vitamins include: I Caps, Maxivision, Ocuguard, Ocuvite, Ocuvite Extra,
Ocuvite PreserVision, Ocuvite Lutein, and Vizion. Supplementation with foods rich
in lutein and zeaxanthin [19] or with lutein rich supplements [21,22] increases macular pigment density in most, but not all, human subjects. Nutritional
supplements are therefore promoted actively by the vitamin industry in the United
States to individuals at risk for AMD. Yet the benefits of ocular antioxidant carotenoid
containing vitamin and mineral supplements in patients with ARM, or those at risk
of developing the disease remain unproved.

The relation of fasting plasma levels of retinol, ascorbate, alpha-tocopherol, beta-carotene
and the use of vitamin supplements has shown that alpha tocopherol, and an antioxidant
index including alpha tocopherol, beta-carotene, and ascorbate were protective for
ARM [43]. However, no evidence of the protective effect of vitamin supplements was found in
this study. In other studies discordant responses of serum and retina to dietary supplementation
were observed. Serum lutein increased rapidly after supplementation in individuals,
but macular pigment density increased only after several weeks of supplementation
[19,22,44]. Some studies have shown no effect of supplementation with purified or synthetic
beta-carotene on the concentrations of several plasma carotenoids [45-48]. In contrast, other studies have shown that purified or synthetic beta-carotene can
diminish concentrations of lutein [49-51]. Hammond et al have reported that out of 13 subjects given supplemental dietary sources
of lutein and zeaxanthin 2 subjects were retinal non-responders- that is, they had
significant increases in serum lutein, but not in macular pigment density [19]. Recently the evidence of a higher incidence of cancer among cigarette smokers who
received beta-carotene supplements in 2 studies [52,53], was reported. Although beta carotene is considered safe because its conversion to
vitamin A is limited [54], the ATBC study reported a significantly higher mortality among treated than non-treated
subjects. To our knowledge published data concerning the toxicity of lutein and zeaxanthin
or the optimum dosage or combination of these antioxidant vitamin/ mineral supplements
is lacking. The Age-Related Eye Disease Study (AREDS), under the auspices of the National
Eye Institute of the National Institutes of Health, is carefully following two large
cohorts of patients with AMD – one group is being treated with an Ocuvite formula,
the other with a placebo. When the results of this study begin to be released in five
to ten years, further guidance may become known.

The impact of zinc and other antioxidants on ARM

There seems to be some support in early clinical and epidemiological studies for the
association of zinc and antioxidant nutrients with ARM. In a small clinical trial,
high-dose zinc supplementation has been reported to reduce the loss of visual acuity
in patients with macular degeneration [55]. Yet other studies were only weakly supportive of the protective effect of zinc on
the development of early ARM [56]. More recently, the Age-Related Eye Disease Study (AREDS), research group found a
beneficial effect for supplementation with a combination zinc, vitamin E, vitamin
C, and beta-carotene in individuals at high risk for disease progression to advanced
AMD [57]. Results of this study showed that for patients at high risk of developing advanced
stages of AMD, use of the combination of antioxidants and zinc supplements reduced
that risk by 25%. For the same group, the risk of vision itself was reduced by 19%.
Prospective studies conducted over time, with greater variability in zinc intake in
the Beaver Dam population are under way and shall provide further estimates on the
protective effect of zinc.

Transport and uptake mechanisms of the carotenoids

The biochemical mechanisms that mediate the selective uptake, concentration, and stabilisation
of the of the macular carotenoids are unknown. In lower animals, such as lobsters
and cyanobacteria, specialised carotenoid-binding proteins perform these tasks. Much
less is known about carotenoid-binding proteins derived from vertebrates, yet it has
been hypothesised that comparable carotenoid-binding proteins may have a similar role
in the human macula. In the human blood stream, high-density lipoprotein (HDL) is
the major carrier of lutein and zeaxanthin, while carotenes are preferentially carried
by low-density lipoprotein (LDL) [58]. In the mammalian eye, it has been reported that retinal tubulin binds macular carotenoids
[59], possibly as a site for passive deposition in the tissue. Further postulations included
the assumption that human macular membranes could be a rich source of specific binding
proteins for the macular carotenoids, especially since many plant and invertebrate
carotenoid-binding proteins are known to be membrane associated [60,61]. In a recent report, xanthophyll-binding proteins (XBP) were partially purified and
isolated from the human macula and retina and it was shown that lutein and zeaxanthin
bind specifically to these proteins [62]. Available evidence further suggests the presence of tissue competition for plasma
carotenoids. The concept that adipose tissue and retina may compete for dietary lutein
has been suggested [44], and the interactions between carotenoids during intestinal absorption has also been
investigated [63]. It has hypothesised that if adipose tissue and liver compete with the retina for
dietary lutein as suggested by observations in human subjects [44], macular pigment may be more effectively increased through supplementation with zeaxanthin
than with lutein (preferentially absorbed by fat). There is no evidence in literature
suggesting that zeaxanthin can be converted to lutein in serum or retina, but the
reverse has been proposed by some authors [64,65]. Dietary lutein may serve as a precursor for the very high concentrations of zeaxanthin
found in the primate fovea, [64] and conversion of lutein to mesozeaxanthin has been suggested [66].

Controversial evidence on the beneficial effects of carotenoids and other antioxidant
vitamins in modulating the course of AMD

Epidemiological evidence

Some epidemiological evidence suggest a beneficial role for carotenoids and antioxidant
vitamins in the prevention of AMD [31,67,68]. In contrast, in a case control study consisting of subjects with late AMD, exudative
AMD, and retinal pigment abnormalities with the presence of drusen, and an equal number
of control subjects, the serum concentrations of lutein and zeaxanthin were found
to be unrelated to the risk of incidence of AMD [69]. Lower risk for macular degeneration has been associated with the consumption of
food sources of these carotenoids [70], with overall level of lutein and zeaxanthin in the diet [68,71], or with higher levels of these carotenoids in the blood [31]. However, in several studies, these associations were not observed [56,72] or were observed only in population subgroups [71]. Other epidemiological studies have reported a protective effect against lung cancer
of foods rich in beta-carotene [73]. This finding has been contradicted in two recent studies in which supranutritional
doses of beta-carotene (25–50 mg/day) supplemented to smokers during 6 y had a higher
incidence of lung cancer than did the placebo-treated control subjects [52,74]. Please see table 1, 1.

Additional File 1. Summary of major epidemiological studies suggesting or contradicting a beneficial
role of the carotenoids or other antioxidants in altering the pathogenesis of macular
degeneration.

Experimental evidence

The body of evidence available from studies in laboratory animals that supports associations
between the intake of antioxidant nutrients and ARM is inconsistent. Several antioxidant
nutrients known to affect retinal degeneration in animals have not been found to consistently
correlate with macular degeneration in humans. In macaque monkeys fed diets devoid
of all sources of carotenoid pigment, levels of these pigments in the macula disappear
and retinal abnormalities (drusen formation), an early sign of ARM appear [75]. In contrast, inverse associations with several carotenoids in the serum [43,69] or diet and early ARM have not been documented in studies in humans.

In a recent study done on rhesus monkeys serum levels and macular density of zeaxanthin
was raised by feeding monkeys a carotenoid-containing fraction of fructus lycii also
known as Gou Qi Zi [76]. This was contrary to the study by Snodderly et al [77] where no change in the concentration of serum lutein was noted after supplementation
of zeaxanthin in squirrel monkeys.

A potential animal model in which to study the protective effects of these carotenoids
is the quail. The quail retina has been shown to selectively accumulate lutein and
zeaxanthin [78,79] and quail retina has been shown to exhibit age-related loss of photoreceptors [80]. Preliminary studies indicate an inverse correlation between the level of zeaxanthin
in quail retina and light-induced cell death [81,82]. Further studies report that quail fed for 6 months on zeaxanthin-supplemented diets
have an almost 5-fold elevation in retinal zeaxanthin, and a more than 45-fold increase
in zeaxanthin concentrations in serum, liver and fat [79].

Reports on other antioxidant vitamins are also controversial. Supplementation of rats
with vitamin C has been reported to reduce retinal damage due to intense light exposure
[83]. However, several other human studies that have examined levels of vitamin C in the
serum [43,67] or foods [68,70] have found similar statistically significant associations. Reports have shown that
animals with diets deficient in vitamin E have increased retinal damage [84], although supplementation with this nutrient does not protect against light damage
[85]. In human studies, one study [43], but not 3 others [67,69,86], have found low levels of this nutrient in the serum to be associated with lower
risk for various forms of ARM.

Clinical evidence

Of the hypothesis that macular pigment protects against AMD was a study in which risk
factors for AMD namely, age and advanced disease in the fellow eye, were shown to
be associated with a relative absence of macular pigment [87]. Recently an autopsy study has reported that eyes from donors with a history of AMD
had lower levels of macular carotenoids than eyes without a known history of AMD [88], although studies relying on post mortem analysis and retrospective reviews of clinical
records after death have substantial limitations.

In some studies MP density was changed by dietary modification or through lutein supplementation
[21,22]. In another study, some subjects failed to show a change in MP density after increasing
their dietary intake of lutein and zeaxanthin [19]. Evidence in favour of changing the dietary intake of fat soluble vitamins to protect
against AMD has been contradicted by some authors [89]. Reports have also been made on the influence of factors such as sex, and smoking
on the relation between MP density and serum concentrations of lutein and zeaxanthin
[90,91], yet these associations were evaluated in small study samples. Please see table 2, 2.

Additional File 2. Summary of major clinical studies suggesting or contradicting a beneficial role of
the carotenoids or other antioxidants in altering the pathogenesis of macular degeneration.

Effect of light and genetic influence on ARM

It has been suggested that the exposure of the retina to light can promote the development
of macular degeneration [92]. Increased risk for late AMD was associated with increased exposure to blue and visible
light in a case control study of Chesapeake watermen [93,94] and with increased exposure to sunlight in the Beaver Dam Eye Study [95]. Light exposure can also increase the production of free radicals in the lens and
retina [96]. Epidemiological data to support a damaging role of light in macular degeneration
is inconsistent [97], although the difficulty in capturing actual light exposure over many years in the
participants studied must be taken into account.

Several results on genetic studies with ARM have been published. First-degree relatives
of ARM patients are between 2 to 4 times greater at risk of developing ARM in comparison
to controls [98]. Twin studies have shown high levels of concordance of the disease among monozygous
sibs [99,100]. Careful segregation analyses on a large study of 564 families suggest that a single
major gene accounts for 89–97% of the genetic variability or 55–57% of the total variability
[101]. Even so, relatively little is known about identified genetic risk factors for ARM
and controversial reports exist supporting genetic variations [102,103].

One of the more promising therapies is photodynamic therapy (PDT). A light activated
drug is administered intravenously, followed by a waiting period of several minutes
to allow the neovascular network of vessels to bind and absorb the drug. Laser light
is then applied to the now pharmacologically-sensitised target tissues resulting in
precise demise of the microvascular network. This process spares the overlying elements,
whereas surgery and radiation could cause retinal scar formation.

Recently, a combination of PDT along with intravitreal steroid (triamcinolone-acetonide)
injections has been introduced. Steroid compounds are well known antiangiogenic agents
which have been suggested as treatment for the choroidal neovascular membranes that
cause visual loss in ARM [104,105]. We hope to release the results of the study done on a cohort of patients treated
with triamcinolone and PDT, along with a control group of patients treated with PDT,
within one year. Much of our hope for successful future treatments also lies in the
areas of suppression or modification of neovascular response with novel anti-antiogenic
factors and gene therapy.

Discussion

The information available provides an indication that the carotenoids, lutein and
zeaxanthin, may play a role in modulating the course of AMD, yet critical evidence
of the beneficial effect has not been found, and crucial information for the most
effective design of clinical trials is needed [106]. For the clinician it is clear that this area of research is only beginning to evolve
and further research is indicated. There is no doubt that any scientific support for
the use of these carotenoids and /or other vitamins or minerals with antioxidant properties
will boost the supply of these supplements on pharmacy shelves, despite the unproved
benefits. As of yet the long-term physiological consequences of taking ocular vitamin
supplements are unknown, and until the optimum combination and dosage of these ocular
antioxidant vitamins and minerals has been proven and their long-term safety established,
the routine prescription of vitamin/mineral supplements cannot be justified.

We finally suggest that clinicians inform their patients that there is no agreement
among scientists and doctors to the benefit of supplementation and we urge them to
be cautious when prescribing ocular vitamin/mineral supplements. However, patients
with ARM or at risk of developing the disease, should be encouraged to eat a balanced
diet rich in fruits and vegetables, and in particular they should be informed by they
clinician on the dietary sources rich in these carotenoids. We further recommend patients
to wear UV protective lenses and a hat or cap when outdoors and suggest they see their
primary care physician to treat any hypertension, hypercholesterolemia or potentially
compromising vascular disease. We truly hope that future investigations on the role
of nutrition, light exposure and genetics help reduce the incidence of this debilitating
disease.

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Mares-Perlman JA, Fisher A, Klein R, Palta M, Block G, Millen AE, Wright JD: Lutein and zeaxanthin in the diet and serum and their relation to age-related maculopathy
in the Third National Health and Nutrition Examination Survey.